The present invention relates generally to protection circuits for rechargeable batteries, and more particularly to a protection circuit for discharging a rechargeable battery when a protection circuit detects a fault condition.
Many portable electronic devices utilize a rechargeable battery to provide power. These devices include computers, cellular telephones, pagers, radios, and the like. While there are many types of rechargeable batteries used today, including nickel cadmium and nickel metal hydride, lithium ion batteries have become a popular choice. Lithium ion batteries are typically smaller and lighter than other rechargeable battery types while charge capacity is increased.
The charging of lithium ion batteries is conducted in a different manner than the charging of nickel type rechargeable batteries. Generally, nickel type rechargeable batteries are charged by applying a constant current until the cell reaches a predetermined voltage or temperature. A lithium ion cell, however, uses a different charging process. First, the lithium ion cell is supplied with a current until the cell""s voltage rises above a threshold. Next, the battery charger is held at the threshold until the current of the cell decreases to a predetermined level.
Therefore, if a lithium ion battery is placed within a charger designed for a nickel rechargeable battery, the result may be damaging to the battery. For example, the voltage of the lithium ion battery may rise to a dangerous level or overheat. If the battery is overcharged a potential for an explosion of the battery exists.
Protection circuits have been developed to prevent such overcharging, but may result in a battery that is unstable and unusable when disposed.
The present invention is directed at providing an apparatus and method that discharges a rechargeable battery when a fault condition has been detected while charging the battery cell. More specifically, if a protection circuit on the battery detects a fault condition, the battery cell is discharged to a safe level.
According to one aspect of the invention, a protection circuit determines when a fault condition occurs during charging. Upon the fault condition, the cell is disabled from being charged further and a discharge circuit discharges the charge stored in the cell.
According to another aspect of the invention, the protection circuit determines when an improper charging condition exists. An improper charging condition may include a charging current or voltage potential being above a predetermined threshold. When the improper charging condition is detected, the charging of the cell is stopped, and the discharge circuit discharges the charge stored within the cell to a safe level.
According to another aspect of the invention, the protection circuit includes a temperature protection circuit that determines when the temperature of the battery is above a predetermined threshold. When the temperature rises above the predetermined temperature the charging of the cell is stopped, and the discharge circuit discharges the charge stored within the cell.